Ok, first you need to decide your variables, then set up your two equations.
Let's use d for dimes, and n for nickels.
The first equation involves what they add up to. A dime is worth $.1 and a nickel $.05.
My first equation looks like this:
.1d + .05n = 6.9
My second equation involves setting up the number of d in terms of n. Translating words into math language can be a bit tricky until you get the hang of it. Where you see "is" put =. So you have d = 14 more than 2 x n.
What that looks like as an equation is this: d = 2n + 14
Then you substitute (2n+14) in for d in the first equation, which is why they gave you two equations - so you could get rid of one of the variables.
.1(2n+14) + .05n = 6.9
simplify by distributing the .1 and combining like terms
.2n + 1.4 + .05n = 6.9
.25n + 1.4 = 6.9
subtract 1.4 from both sides
.25n = 5.5
divide both sides by .25
n = 22
Substitute back into second equation and solve for d:
d = 2n + 14
d = 2(22) + 14
d = 44+14 = 58
Check work
.1(58) + .05(22) = 6.90
5.8 + 1.1 = 6.90
6.9 = 6.9
That works!
(SIDE NOTE: When you're dealing with 14 more than, it doesn't matter if you put 14 + 2n or 2n + 14. But when you are dealing with less than, it does. You have to subtract the number afterward - so 14 less than twice the number of nickels would be 2n - 14)
